Typically, a brake pressure control system of this type is an anti-lock system. However, it is also desired to include all other control systems wherein the pressure in the wheel brake can be adjusted independently of the pedal force. One type of a brake pressure control system is referred to as fluid-return principle. A system of this type includes an inlet valve which can close the connection between the master cylinder and the wheel brake cylinders, and an outlet valve which connects the wheel brake cylinder to a low pressure accumulator. The pressure fluid discharged into the low pressure accumulator is returned by a return pump.
The brake pressure can be controlled by opening and closing the inlet valve and the outlet valve according to the specifications of an electronic control system.
Usually, pressure fluid remains in the low pressure accumulator after such a control operation has been completed. This pressure fluid must be removed in a so-called follow-up period of emptying period and returned into the supply reservoir of the system. Typically, the control of a system of this type arranges for a follow-up of the motor which drives the return pump for a defined interval after a braking operation has been terminated (which can be identified by monitoring the brake light switch).
The interval is rated so that even a completely filled low pressure accumulator can reliably be emptied in any case. This condition may be easily realized in terms of circuit technology. However, the disadvantage is that during each braking operation, where brake pressure control has been activated, the motor or the pump still produces noise even after termination of the control for a relatively long interval.
An object of the present invention is to minimize noises. To this end, the present invention discloses monitoring the rotational speed of the pump in the emptying period and disconnecting the pump and terminating the emptying period due to defined criteria with respect to the rotational speed.
This provision permits adapting the emptying time to the actual filling level of the low pressure accumulator. Normally, the emptying time is this way shortened so that disturbing noise is also minimized.
A monitoring operation of this type can be achieved most easily by actuating the motor which drives the pump in a pulse-width modulated fashion in the emptying period, and the rotational speed is measured in the sections of the control signal for the pump where no voltage is applied to the motor.
A simple magnitude which represents the rotational speed of the motor and the pump connected to the motor is achieved by measuring the electromotive counterforce.
The rotational speed itself can be taken into account as a criterion for disconnection of the pump, and the emptying period is terminated by disconnection of the motor as soon as a defined rotational speed is exceeded.
However, it is also possible to take into account the rotational speed increase from measuring point to measuring point, and the emptying period is also terminated after a defined threshold has been exceeded.